1. Field of the Invention
The present invention relates to a device for determining the deterioration of an air-fuel (A/F) ratio sensor employed in an air-fuel ratio control device of an internal combustion engine in order to control the air-fuel ratio (i.e., the ratio at which air and fuel are mixed together) at a desired value, by supplying fuel in accordance with the amount of air intake More specifically, the present invention relates to a device for determining the deterioration of an A/F ratio sensor located upstream of an exhaust gas purification catalyst for A/F feedback control, the A/F ratio sensor being capable of linearly detecting the air-fuel ratio.
2. Description of the Related Art
Conventionally, automobile engines employ a three-way catalyst as means for purifying their exhaust gas, where the three-way catalyst simultaneously promotes the oxidation of the unburnt components (hydrocarbons (HC), and/or carbon monoxide (CO)) and the reduction of nitrogen oxides (NO.sub.x). In order to enhance the oxidation and/or reduction ability of the three-way catalyst, it is necessary to control the A/F ratio (which indicates the state of combustion within the engine) to be in the vicinity (or "window") of a theoretical or stoichiometric air-fuel ratio Thus, the fuel injection control for an engine is typically achieved by means of an O.sub.2 sensor (FIG. 1) for detecting whether the A/F ratio is "rich" (implying a relatively large supply of fuel) or "lean" (implying a relatively small supply of fuel) as compared to the stoichiometric A/F ratio, so that the amount of fuel supplied is corrected based on the detected residual oxygen concentration in the exhaust gas (i.e., the output of the O.sub.2 sensor).
In recent years, internal combustion engines have been developed which are capable of controlling the A/F ratio so that the three-way catalyst therein can maintain a constant purification ability. Behind this concept is a fact that a three-way catalyst, which adsorbs oxygen within exhaust gas when the A/F ratio of the exhaust gas is in a lean state and releases the adsorbed oxygen when the A/F ratio of the exhaust gas is in a rich state (such function is referred to as "O.sub.2 storage function"), has a limited O.sub.2 storage capability. Therefore, in order to allow a three-way catalyst to fully utilize its O.sub.2 storage capability, it is essential to maintain the amount of oxygen stored in the catalyst at a predetermined value, e.g., half of its maximum oxygen storage capacity, so that the three-way catalyst is kept ready for an on-coming lean condition or rich condition in the A/F ratio of the exhaust gas. By thus maintaining the amount of oxygen stored in the catalyst at such a predetermined value, the three-way catalyst can exhibit constant O.sub.2 adsorbing or releasing capabilities, and hence constant oxidation or reduction capabilities.
Such an internal combustion engine capable of controlling the amount of O.sub.2 stored in a catalyst for maintaining the purification capabilities of the catalyst typically employs an A/F ratio sensor. Such an A/F ratio sensor exhibits a characteristic curve as shown in FIG. 2, and is capable of linearly detecting a broad range of A/F ratios including the stoichiometric A/F ratio. Specifically, the A/F ratio sensor is employed for attaining a feedback control which is based on proportional and integral (PI) operations that can be expressed as follows: EQU (Next fuel injection correction amount)=K.sub.p .times.(fuel difference in the current session)+K.sub.s .times..SIGMA.(fuel differences in all past sessions)
where the term "fuel difference" is defined as:
(the amount of fuel actually burnt within cylinders)--(the target fuel amount to be burnt within cylinders with a stoichiometric amount of air intake), where the term (the amount of fuel actually burnt within cylinders) is defined as (detected air amount)/(detected A/F ratio); PA1 the coefficient K.sub.p represents a gain for the proportional term; and PA1 the coefficient K.sub.s represents a gain for the integral term.
Thus, the fuel injection correction amount is constantly calculated in the context of feedback control.
As seen from the above, the equation for calculating the fuel injection correction amount includes a proportional term prefixed by the coefficient K.sub.p and an integral term prefixed by the coefficient K.sub.s. The proportional term is a component for maintaining the A/F ratio at the stoichiometric A/F ratio, whereas the integral term is a component for eliminating an offset The integral term serves to maintain the amount of O.sub.2 stored in the catalyst at a constant value. For example, if lean gas is generated in response to an abrupt acceleration, the integral term causes rich gas to be generated so as to cancel the effect of the lean gas.
As described above, the A/F ratio feedback control which is based on the output voltage of an A/F ratio sensor is performed so as to increase the fuel injection correction amount as an offset of the output voltage from a target voltage (i.e., a voltage corresponding to the stoichiometric A/F ratio) increases. However, as the A/F ratio sensor deteriorates due to the heat of the exhaust gas and/or the poisonous effects of the lead component, phosphorus component, etc. within the fuel and/or lubrication oil, the response characteristics (i.e., reaction speed at which the sensor can follow actual changes in the A/F ratio) of the A/F ratio sensor decreases, thereby making it difficult to achieve the desired A/F ratio feedback control.
A conventional device for detecting the deterioration of an A/F ratio sensor is disclosed in, for example, Japanese Laid-Open Patent Publication No. 5-106486. The disclosed device for determining the deterioration of an A/F ratio sensor relies on the output of an A/F ratio sensor that is capable of continuously detecting the A/F ratio, which may take any value within a broad range of A/F ratios including the stoichiometric value. The device learns respective feedback correction mounts for a target A/F ratio set at the stoichiometric A/F ratio and for a target A/F ratio set at a value different from the stoichiometric A/F ratio (e.g., a lean A/F ratio) based on the output of the A/F ratio sensor, and determines the deterioration of the A/F ratio sensor based on a difference between the respective learned values.
Since the above-described conventional device for detecting the deterioration of an A/F ratio sensor must learn the feedback correction amounts associated with different target A/F ratios (i.e., the stoichiometric value and another value) and compare the learned values, there is a disadvantage in that the deterioration determination often takes a long times Furthermore, the determination of A/F ratio sensor deterioration by the conventional device is only applicable to an A/F ratio control system whose control is directed to both the stoichiometric A/F ratio and another A/F ratio, erg., a lean A/F ratio. That is, the conventional device is not applicable to a system where the A/F ratio is always controlled toward one target A/F ratio (erg., the stoichiometric A/F ratio). Moreover, the conventional device relies on the fact that a relatively large fluctuation occurs in the output of deteriorated A/F ratio sensors while performing an A/F control on the lean side (as opposed to the stoichiometric A/F ratio) That is, the determination is dependent on the deterioration or fluctuation that occurs in only a limited control range of the A/F ratio sensor, thereby making it difficult to provide a stable determination of deterioration.